1. BMFS 3373 CNC TECHNOLOGY Lecture 10

2. Lecture Objectives At the end of the lecture, you will be able to:
Explain the different types of positioning modes for CNC lathe operation
Know the important locations for programming and setup which includes reference point, machining origin and program origin
Know the important preparaotory (G) codes and miscelleneous (M) codes used in programming lathe operations
State the codes for specifying values of the spindle speed and tool feed.
Explain the tool nose radius compensation

3. Cartesian Coordinates in CNC Lathe
CNC lathe program can be written to move the tool in the following modes:
Absolute Positioning
Incremental Positioning
Mixed (Absolute & Incremental) Positioning

4. Cartesian Coordinates in CNC Lathe
Absolute Positioning:
The new position of the tool is given by its X and Z distances from a fixed home or origin (0,0)
the X position of the tool is specified in terms of diameter or twice the distance from the spindle centerline.
Incremental Positioning:
The new position of the tool is specified by inputting its direction and distance from the last position achieved.
the U is entered as the directed change in diameter from last positioned achieved.

5. Cartesian Coordinates in CNC Lathe
Absolute Positioning
Incremental Positioning

6. Feature Location – Machine Setup

7. Feature Location – Machine Setup
Reference Point (Machine Zero)
The position of the turret when the machine’s axes are zeroed out
It is set once by the manufacturer
Also known as Machine Home
Machining Origin (Tool Change Position)
It is determined at setup and is input at the beginning of the word address program before the first tool change by using a zero-offset command.
The machine indexes the tools at this position.
It is position when the turret is set with its longest tool at least 1.0 from the face of the part and 1.0 from the diameter of the stock. Then the control is zeroed out at this location
The turret is returned to the reference point with the relative distance of the two points (reference & machining) is recorded

8. Machining Origin Word Address Program: N0020 G50 X10.Z5. N0030 T0101
Set Machining Origin at X10.Z5. before tool change position (T0101)

9. Feature Location – Machine Setup
Program Origin(Program Zero)
The point at which all dimensions are defined in the part program.
The setup uses tool offsets as a mean of locating the program zero with respect to the machining origin. The controller will compute the corresponding X and Z move relative to the machining origin. It will then execute the move relative to the machining origin.

10. Program Origin
Manual Touch Off Method

11. Program Origin
Q-Setter / Tool Setter Method

12. Setup Procedure
The setup operation normally begins with the setup person securing the required OD and ID cutting tools in the turret as specified by the operation sheet.
The setup person must measure and enter the value of the variety tool length offsets or geometry offsets of each tool into the system memory. The system will compensate these variation by analyzing the initial distance between the tip of the tool and the program zero.
There are two methods where tool offsets are recorded:
Manual Touch Off
Q-Setter
Another concern during machining is the tool wear. To overcome this, the operator opens the wear offset page and enters the tool wear offset to compensate the differences. This is done during production machining in order to meet tolerances

13. Setup Procedure
Manual Touch Off Method

14. Setup Procedure
Q-Setter / Tool Setter Method

15. Important Lathes Preparatory Functions

16. Tool Edge Programming
It is in effect when offsets are measured to the edge of the tool.
With this programming, the part geometry can be input directly without the consideration of the cutting tool nose radius.
Thus it can only applied to part geometries consisting of horizontal or vertical lines. Other geometries will require additional compensation program known as tool nose radius (TNR) compensation.

17. Tool Nose Radius Compensation

18. Tool Nose Radius Compensation

19. Tool Nose Radius Compensation
To setup Tool Nose Radius (TNR) Compensation, the control is instructed to open an offset file in memory to key-in the following information:
X and Z tool offsets to the tool edge
Size of the tool nose radius
Tool nose vector

20. Tool Nose Radius Compensation
Tool Nose Vector
The tool tip (imaginary tool point) of a single point tool has a specific location from the center of the tool nose radius.
The tool nose vector indicates this location to the controller.
The controller will use this information to properly determine the tool movement in response to a compensation command

21. Tool Nose Vector

22. Tool Nose Radius Compensation
Some restriction when handling a tool nose radius compensation:
A G code activating TNR compensation is entered as a separate block in the program. It must be commanded before the tool starts to cut.
The first and second block following a TNR compensation code must contain XZ linear motion command that signals the initiation or termination of the compensation. Failure to do so, will result in over or undercutting
Motion in the G40, G41 or G42 block must be greater than twice the value of the tool nose radius that will ensure no over or undercutting to happen

23. Tool Nose Radius Compensation

24. Tool Nose Radius Compensation
A G41 or G42 indicates TNR compensation is start up on the next linear move.
The MCU transfers the tool positioning point from the imaginary point A as shown in Fig and to the center of the tool nose radius.
The controller will offset the values with respect to the inputs stored in the memory placed during setup.
A G40 will cancel out the compensation, and the MCU will shift back the tool position point back to the imaginary point A.

25. End
Chapter 10